1. Field of the Invention
This invention relates to improved polypropylene compositions containing very small amounts of zinc oxide as a multipurpose additive which acts to make the composition non-corrosive to metal equipment during thermal processing and to prevent plate-out of low molecular weight components on processing equipment during thermal processing.
2. Description of the Prior Art
Polypropylene produced by polymerization of propylene in the presence of so-called Ziegler of Ziegler-Natta catalysts is a well established material of commerce. Commercial polypropylene contains a high proportion of polymer of sterically regular structure and high molecular weight, commonly referred to as "isotactic" polymer, which is insoluble in boiling hexane or heptane and in xylene at ambient temperatures. Commercial polypropylene typically also contains small proportions of material of relatively low molecular weight and material of irregular steric structure, known as atactic polypropylene. Some of these components are soluble in boiling hexane or heptane, and some additional non-isotactic polymer is soluble in xylene at 25.degree. C. Commercial polymer further usually contains small amounts of catalyst residue which includes chloride ions.
It is conventional to stabilize polypropylene against degradation under the influence of heat during processing. Such conventional stabilization systems may contain a phenolic antioxidant together with a synergist such as dialkylthiodipropionate. Polypropylene which is to be employed in outdoor exposure is also conventionally stabilized against deterioration under the influence of actinic light.
In the thermal processing of polypropylene which contains only a heat stabilization system, the presence of chlorine-containing catalyst residue causes liberation of hydrogen chloride during processing with resulting corrosion of metal equipment with which the polymer is in contact, such as molds, extruders, chill-rolls or the like. It has become conventional practice to add calcium stearate to polypropylene compositions as an acid acceptor in concentrations of typically 0.1 to 0.25%.
Compositions containing calcium stearate, particularly those which also contain a relatively high proportion of non-isotactic xylene-extractable components, have the disadvantage that some of the stearate, during processing, may be converted to stearic acid which migrates to the surface and appears to facilitate the migration to the surface of the lower molecular or atactic components of the polypropylene. A similar effect is observed in compositions in which organic stearates, i.e., esters of stearic acid, are present as so-called antistatic additives. The migration of low molecular weight or atactic polymer to the surface during processing results in so-called plate-out of the migrated polymer, i.e. deposition of such low molecular weight and atactic material on the processing equipment, which interferes with proper operation of the equipment. Plate-out is particularly objectionable, for example, in the casting of films, where it deposits on the chill-rolls, and in injection molding where it deposits on the molds.
In polypropylene containing calcium stearate as acid acceptor it is also found that migration of stearic acid to the polymer surface during processing at high temperatures results in some corrosion of processing equipment.
Another situation in which migration of low molecular weight or atactic material or of stearic acid from polypropylene is objectionable is when such polymer is used in the interior of automobiles, where the material vaporizing from the polymer tends to condense on glass surfaces such as the windshield.
Migration of low molecular weight or atactic material or of stearic acid to the surface is also objectionable in polypropylene articles which are to be decorated by printing and, in general, in articles in which a highly glossy surface is desired.
As will be shown below, the present invention is based on the discovery that the presence of small amounts of finely divided zinc oxide in polypropylene not only substantially eliminates corrosivity of the composition, but also prevents undesired plate-out on processing equipment. Zinc oxide used according to the invention also has other incidental beneficial effects, such as a synergistic action with antioxidant and UV stabilization systems.
Zinc is a metal of Group II of the Periodic Table. In 1955, before polypropylene was a material of commerce, it was suggested to add Group II metal compounds, including oxides, to polyolefins containing Ziegler catalyst residue.
U.K. Pat. No. 830,924 to Farbwerke Hoechst, based on a filing in Germany on Apr. 4, 1955, teaches the addition of 0.01 to 3% of inorganic salts or oxides of a metal of Group IIA, IIB or IVB of the Periodic Table to polyethylene to prevent the discoloration and emission of unpleasant odors which occurs during molding of polethylene in the absence of such an additive. The patent also discloses that such inorganic compounds diminish the corrosion of the metal parts that come into contact with the hot plastic mass during processing. The only specific examples in the patent application employ 0.5% of magnesium carbonate and 0.8% of calcium oxide, respectively. Suitable compounds enumerated in the disclosure are sulfates, phosphites, carbonates and oxides of calcium, cadmium, barium, lead, magnesium and zinc.
U.K. Pat. No. 819,277 to The Dow Chemical Company, based on an application filed in the U.S. on Dec. 19, 1955, discloses the addition to olefinic materials, polymerized with Ziegler catalyst, of a corrosion-inhibiting quantity of a basic inorganic compound of a Group II metal of the Periodic System which has a receptor function for and is capable of counteracting hydrogen halide and like corrodents. The disclosure is essentially in terms of polyethylene. The patent discloses addition of the corrosion-inhibiting material in amounts of 0.005 to 5% by weight and suitably 0.5 to 2% by weight. The basic compounds disclosed as useful are the oxides and hydroxides of the Group II metals. It is stated that the metals are "calcium, magnesium, strontium, barium and zinc, in descending order of desirability". A single example illustrates the effect on the corrosivity of polyethylene of adding, respectively, 1.5% by weight of calcium oxide, magnesium oxide, barium oxide and zinc oxide and 2% of calcium hydroxide. Evidence of corrosion with the calcium compounds is stated to be "none" and with the magnesium, barium or zinc oxides "neglible".
In spite of these early disclosures bearing on the use of zinc oxide as halogen acceptor, the industry did not turn to the use of zinc oxide but appears to be generally employing calcium stearate as acid acceptor in polypropylene produced with Ziegler catalysts. However, a commercial use of zinc oxide in polypropylene is as a ultraviolet screener, employed in concentrations of 3 to 10 phr (parts per hundred parts of resin) together with a synergist. Several materials are known as synergists for zinc oxide in this use, including titanium dioxide and zinc dithiocarbamates.
Other uses for zinc oxide in polyolefins, disclosed in the patent literature, are in the stabilization of ethylene-propylene copolymers, using 0.1 to 5% of zinc oxide in combination with sulfur or other Group VI elements, the zinc oxide absorbing the H.sub.2 S or equivalent, liberated during thermal processing (U.S. Pat. No. 3,111,499); as one of a large group of organic and inorganic bases employed in combination with thiophosphites or the like to provide thermal and light stabilization (U.S. Pat. No. 3,179,621); as one of the group of "anionic exchange or basic substances" including oxides, hydroxides or longe chain fatty acid salts of alkaline earth metals, tin and zinc, metal soaps being preferred, in combination with an acidic ultraviolet-absorbing agent to provide UV stability (U.S. Pat. No. 3,238,163); as one of the salts, acids, oxides, soaps and hydroxides of 21 named metals to improve the corrosivity of polyolefins (U.S. Pat. No. 3,331,806); in concentration of 0.25 to 10%, together with zinc dibutyldithiocarbamate with either 2,6-didodecyl-p-cresol or dilaurylthiodipropionate to provide a white, light stable composition (U.S. Pat. No. 3,376,250); in concentrations of 10 to 100% as filler for polypropylene (U.S. Pat. No. 3,462,389); and as one of a group of inorganic fillers in concentration from 50 to 1900% based on polypropylene (U.S. Pat. No. 3,745,142).